2004A&A...422..925S

Between 1996 and 2002, we have carried out a spectral monitoring program for the Seyfert galaxy NGC 5548 with the 6m and 1m telescopes of SAO (Russia) and with the 2.1m telescope of Guillermo Haro Observatory (GHO) at Cananea, Mexico. High quality spectra with S/N>50 in the continuum near Hα and Hβ were obtained, covering the spectral range ~(4000-7500)Å with a (4.5 to 15)Å-resolution. We found that both the flux in the lines and the continuum gradually decreased, reaching minimum values during May-June 2002. In the minimum state, the wings of Hβ and Hα became extremely weak, corresponding to a Sy1.8 type, not to a Sy1, as observed previously when the nucleus was brighter. The line profiles were decomposed into variable and constant components. The variable broad component is well correlated with the continuum variation. It consists of a double peaked structure with radial velocities ~±1000km/s relative to the narrow component. A constant component, whose presence is independent of the continuum flux variations, shows only narrow emission lines.The mean, rms, and the averaged over years, observed and difference line profiles of Hβ and Hα reveal the same double peaked structure. The relative intensity of these peaks changes with time. During 1996, the red peak was the brightest, while in 1998-2002, the blue peak became the brighter one. Their radial velocities vary in the ~(500-1200)km/s range. In 2000-2002 a distinct third peak appeared in the red wing of Hα and Hβ line profiles. The radial velocity of this feature decreased between 2000 and 2002: from the observed profiles, from ~+(2500-2600)km/s to ~+2000km/s and is clearly seen on the difference profiles. The fluxes of the various parts of the line profiles are well correlated with each other and also with the continuum flux. The blue and red parts of the line profiles at the same radial velocities vary in an almost identical manner. Shape changes of the different parts of the broad line are not correlated with continuum variations and, apparently, are not related to reverberation effects. Changes of the integral Balmer decrement are, on average, anticorrelated with the continuum flux variations. This is probably due to an increasing role of collisional excitation as the ionizing flux decreases. The behavior of the Balmer decrement of the various parts of the line profiles was different in 1996-2000 as compared with the 2001 behavior. Our results favor the formation of the broad Balmer lines in a turbulent accretion disc with large and moving ``optically thick'' inhomogeneities, capable of reprocessing the central source continuum.